The present invention relates to electrically tunable devices particularly for microwaves, which are based on a ferroelectric structure.
Known electrically tunable devices, such as capacitors (varactors) and which are based on ferroelectric structures do indeed have a high tuning range but the losses at microwave frequencies are high thus limiting their applicability. Typical ratios between the maximum and the minimum values of the dielectric constant (without and with applied electric fields) ranges from n=1.5 to 3 and the loss tangents ranges from 0.02 to 0.05 at 10 GHz. This is not satisfactory for microwave applications requiring a low loss. Then e.g. a quality factor of about 1000-2000 is needed. WO 94/13028 discloses a tunable planar capacitor with ferroelectric layers. However, the losses are high at microwave frequencies.
U.S. Pat. No. 5,640,042 shows another tunable varactor. Also in this case the losses are too high Losses across the interface dielectric material-conductor are produced which are high and furthermore the free surface between the conductors results in the ferroelectric material being exposed during processing (e.g. etching, patterning) which produce losses since the crystal structure can be damaged.
What is needed is therefore a tunable microwave device having a high turning range in combination with low losses at microwave frequencies. A device is also needed which has a quality factor at microwave frequencies such as for example up to 1000-2000. A device is also needed in which the ferroelectric layer is stabilized and a device which shows a performance which is stable with the time, i.e. the performance does not vary and become deteriorated with time.
Furthermore a device is needed which is protected against avalanche electric breakdown in the tunable ferroelectric material.
Further yet a device is needed which is easy to fabricate. A device is also needed which is insensitive to external factors as temperature, humidity etc. Therefore an electrically tunable device, particularly for microwaves, is provided which comprises a carrier substrate, conducting means and at least one tunable ferroelectric layer. Between the/each (or at least a number of) conducting means and a tunable ferroelectric layer a buffer layer structure is provided which comprises a thin film structure comprising a non-ferroelectric material.
According to one embodiment the thin film structure comprises a thin non-ferroelectric layer. In an alternative embodiment the thin film structure comprises a multi-layer structure including a number of non-ferroelectric layers. In still further embodiments a multilayer structure including a number of non-ferroelectric layers arranged in an alternating manner with ferroelectric layers (such that a non-ferroelectric layer always is provided adjacent the/a conducting means.
In a particular embodiment the ferroelectric layer is arranged on top of the carrier substrate and the non-ferroelectric thin film structure, including one or more layers, is arranged on top of the ferroelectric layer the conducting means in turn being arranged on top of the non-ferroelectric structure. In an alternative embodiment the ferroelectric layer is arranged above the non-ferroelectric structure including one or more non-ferroelectric layers, which is arranged on top of the conducting means. The conducting means particularly comprise (at least) two longitudinally arranged electrodes between which electrodes or conductors a gap is provided. According to different embodiments the non-ferroelectric structure is deposited in-situ on the ferroelectric layer or deposited ex-situ on the ferroelectric layer.
The deposition of the non-ferroelectric layer may be performed using different techniques such as for examples laser deposition, sputtering, physical or chemical vapour deposition or through the use of sol-gel techniques. Of course also other techniques which are suitable can be used.
Advantageously the ferroelectric and the non-ferroelectric structures have lattice matching crystal structures. The non-ferroelectric structure is particularly arranged so as to cover also the gap between the conductors or the electrodes. In a particular implementation the device comprises an electrically tunable capacitor or a varactor.
In another embodiment the device includes two layers of ferroelectric material provided on each side of the carrier substrate and two conducting means, non-ferroelectric thin film structures being arranged between the respective ferroelectric and non-ferroelectric structures in such a way that the device forms a resonator. According to different implementations the device of the invention may comprise microwave filters or be used in microwave filters. Also devices such as phase shifters etc. can be provided using the inventive concept.
Different materials can be used; one example of a ferroelectric material is STO (SrTiO3). The non-ferroelectric material may for example comprise CeO2 or a similar material or SrTiO3 which is doped in a such a way that it is not ferroelectric. An advantageous use of a device as disclosed is in wireless communication systems.